* Clinton set to meet Lavrov in Washington on Thursday
* Progress on arms control
* Tensions remain over Georgia
WASHINGTON, May 6 (Reuters) - The "reset" button was pushed two months ago on U.S.-Russia ties but results so far have been mixed, with arms control the most promising area but gnarly issues such as Georgia tensions still lingering.
Secretary of State Hillary Clinton is set to see Russian Foreign Minister Sergei Lavrov in Washington on Thursday, their second major meeting since ice-breaking talks in Geneva in March when they symbolically pressed a reset button to turn a page on caustic relations under the former Bush administration.
Since the Geneva meeting, a senior U.S. official said signals coming from Moscow were uneven and it was unclear how far Russia wanted to go in improving ties.
"The challenge now is whether we can put meat on the bones and fulfill the promise of the positive part of the relationship, leading with arms control," the official said.
"Can the problems and areas of disagreement be managed or do they overwhelm it (the relationship)?" he told Reuters.
Renewed tensions this week in Georgia, with Tbilisi accusing Russia of being behind a failed mutiny at a military base, could overshadow the diplomats' talks at the State Department.
Russia fought a war with neighboring Georgia last year which exacerbated tensions with Washington and its allies and sunk relations to a post Cold War low.
""We don't want to have a confrontation over it," said the senior U.S. official. "But beyond the question of Georgia, there is a larger issue of Russia's claim to a sphere of influence."
Russia expert Dmitri Trenin, director of the Moscow center for the Carnegie Endowment for International peace, predicted bullets could "start flying" again in Georgia soon.
"Unfortunately, the prevailing view is that we (the United States) somehow left Georgia behind and that Georgia is not a burning issue, not an issue at the top of the agenda. But Georgia may climb its way back," Trenin said.
Another issue is increased bickering between Moscow and NATO following the expulsion last week of two Brussels-based Russian diplomats from the military alliance.
Lavrov responded by dropping plans to attend a meeting of the NATO-Russia Council this month, a move that will have disappointed Clinton who pushed for such talks to resume after they were suspended following the Georgia war.
The senior U.S. official said Lavrov's response was seen as "odd" and Washington was puzzled.
"The question is are the Russians capable of accepting outreach," the official said.
Russia expert Charles Kupchan, a senior fellow at the Council on Foreign Relations, said the new U.S. approach needed to see some concrete results.
"I think it is important for (President Barack) Obama to begin to be able to show the pay-offs of his public diplomacy and for reaching out to the Russians," said Kupchan.
"There is a bit of a tango at work here. Washington is putting on offer a new relationship and now it behooves the Russians to reciprocate."
Experts say the best chance for early success is in negotiations to replace a Cold War arms reduction treaty, called START 1, that expires in December and which both sides say they are determined to get by the deadline.
The chief U.S. negotiator, Rose Gottemoeller, has indicated some flexibility, telling Russia's news agency Interfax this week that Washington was ready to count both nuclear warheads and their delivery vehicles -- a sticking point previously.
The first round of serious negotiations is set to start in Moscow on May 18, with the goal of having the outlines of a deal when Obama meets Russia's leader in July.
"Certainly, the Obama administration brings a fundamentally different view on arms control. It is prepared to negotiate further nuclear reductions in a framework that is familiar to the Russians," said Steve Pifer, former U.S. ambassador to Ukraine now with the Brookings Institution.
"Strategically when there is a good nuclear dialogue it has tended to have a positive impact on the broader relationship," he added.
But Kupchan pointed to domestic obstacles from the Russian side, with the financial crisis making it more difficult to move towards a rapprochement with Washington.
"During tough economic times, leaders tend to play the Populist card. In the case of Russia, the Populist card generally means drumming up some anti-American sentiment," said Kupchan. (Reporting by Sue Pleming; editing by Vicki Allen)
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